Peripheral blood mononuclear cells (PBMC) from 40 normal individuals, 20 men and 20 women, divided according to age were evaluated for their ability to mediate natural killer (NK) cytotoxicity, using the K562 erythroleukemic cells as target. The results of a specific 51Cr release assay demonstrated that NK activity was greater in men than in women. There was a significant correlation between NK activity and the percentages of Leu 11a+ cells in younger women, but not in the other groups and no correlation between NK activity and Leu 7+ cells